Many of the games actually have more than one leitmotif. AC04 has the melody from "ISAF", also heard in "Operation Bunker Shot" and "Heaven's Gate", in addition to the more common main theme heard in a ton of tracks throughout the soundtrack.
AC5 has the Razgriz theme, the Grabacr theme, and you could also count the variations of the Scinfaxi theme.
ACZ has two recurrent themes, though one could be considered more "main" than the other. Some prominent examples would be this, this and this. "The Valley of Kings" has an off-key, more somber version of it. The secondary theme can be heard in the background here, and is also heard through most of "Diapason" before the main theme kicks in. Likewise, you can also spot a darker version of it in "The Inferno".
AC6 has three recurrent themes. Interestingly enough, the officially called "Ace Combat 6 Main Theme" isn't really the most recurrent theme in the soundtrack, though it does feature in a subdued form in a few tracks (mostly cutscenes), and more prominently as the main melody of "Chandelier" and "Sortie 5". The much better known theme is, of course, the "Gracemeria" theme, which features in a bunch of other tracks beyond "Invasion" and "Liberation" such as "Weapons of Mass Destruction". The third theme is less prominent, but still features in tracks such as most of the "Briefing" tracks, "Malebolge" and builds up through most of "The Moloch Desert".
Edited by PositronCannon, 24 September 2015 - 09:42 PM.